Innovative Boiler Design to Reduce Capitel Cost and Construction Time

Transcription

1 s Innovative Boiler Design to Reduce Capitel Cost and Construction Time Presented originally at Power-Gen 2000 Authors: Joachim Franke Rudolf Kral Power for Generations Siemens Power Generation

2 Content Introduction 3 Power Plant Design 3 Once-Through Operation 6 Vertical BENSON Evaporator Design 7 Horizontal Furnace 10 Studies for the New Concept Concentrate on Four Aspects 10 Summary 12 2

3 Innovative Boiler Design to Reduce Capital Cost and Construction Time Joachim Franke, Rudolf Kral Siemens Power Generation Introduction The requirements to be met by the next generation of power plants are determined on the basis of various criteria. The most important factors on the assessment scale are efficiency and environmental protection, operational flexibility and power generation costs. The steam generator is of major significance as no other power plant component is more cost-intensive or has such a major impact on availability. The economic efficiency of steam power plants is enhanced by elevating steam parameters to the supercritical level, making the use of once-through boilers necessary. Among the various types of once-through boilers, the BENSON boiler for which Siemens is licensor exhibits clear advantages based on its suitability for flexible operating modes with rapid load changes and brief start-up times. The market demand for a reduction in manufacturing and installation costs led to the development of a new BENSON boiler concept incorporating a furnace with vertical tubing. Power Plant Design In recent years, the requirements for coal-fired power plants in countries outside of Europe have also shifted from base load towards intermediate peaking and peak load duty. Frequent startups and shutdowns as well as rapid load changes necessarily lead to a change in operating mode from constant to variable pressure. The reason for this is that under such conditions, large temperature changes and hence high material loads on the HP turbine can be prevented only with variable-pressure operation (Fig. 1). In addition, omission of the complex turbine control stage is only possible with variable-pressure operation. Figure 1 Comparison of different operation modes 3

4 The transition to variable pressure has also had a decisive influence on boiler design as the main steam pressure, which must be increased to over 200 bar in order to enable achievement of higher efficiencies (Fig. 2). Different requirements thus also lead to different power plant concepts (Fig. 3). While all available types of boilers and turbines can be implemented for baseload operation, for intermediate peaking and peak load duty only once-through boilers suitable for variable-pressure operation remain. Various typical power plant types have established themselves based on the differing requirements in force in the various regions (Fig. 4). Previously, only base-load plants of intermediate efficiency were required in the USA as well as in Southeast Asia. Drum-boiler technology is correspondingly widespread in these areas. Figure 2 Efficiency potential of advanced coal-fired power plants Figure 3 Power plant design is defined by the requirements 4

5 A start was made along the road to higher pressures and thus to higher efficiencies in the USA with a boiler technology requiring supercritical pressure in the evaporator over the entire load range. For intermediate peaking duty this method has the disadvantage that part-load operation either entails a significant reduction in plant efficiency with throttling downstream of the evaporator, turbine in variable-pressure operation or that the number or rates of load changes must be significantly reduced no throttling downstream of evaporator, turbine with control stage in fixed-pressure operation. In Europe and Japan, the combined requirement of higher efficiency and more operating flexibility in intermediate peaking and peak load duty lead to the early implementation of once-through boilers with supercritical steam parameters and suitability for variable-pressure operation. The availability of such plants with supercritical pressures is on the same level as that of subcritical plants (Fig. 5), and the large number of reference plants also demonstrates the maturity of this technology. Figure 4 Typical power plant design Figure 5 Energy unavailability not postponable (EU) of German power plants 5

6 Once-Through Operation With approx. 1,000 units constructed, the BENSON boiler is the once-through boiler with the greatest representation worldwide. The various types of once-through boilers differ primarily in their evaporator systems (Fig. 6). Since the 60s, the evaporators in BENSON boilers have been equipped with tubes welded to form membrane walls in a spiral configuration around the furnace. This method, which has since been adopted by most boiler manufacturers worldwide, enables parallel upward flow at high mass flux through all evaporator tubes, and can thus be operated at both subcritical and supercritical pressures. In contrast, the Universal Pressure (UP) or Combined Circulation systems introduced in the USA in the 50s with vertical-tubed evaporators and higher mass velocity are suitable only for operation with supercritical pressure in the evaporator. Variable-pressure operation is possible only by throttling downstream of the evaporator and is thus associated with efficiency losses. The development of new once-through evaporator concepts with vertical tubes began in the 80s. A concept developed by MHI is characterized by a mass velocity in the evaporator tubes which is reduced but is still more than double that in natural-circulation systems, and by an additional evaporator in the convection section of the boiler. In contrast, the Siemens development of vertical evaporator tubes is characterized by low mass velocities comparable with those in natural-circulation boilers and the simplest possible structural design. Figure 6 Once-through steam generators 6

7 Vertical BENSON Evaporator Design Furnaces with spiral-wound tubes can look back on more than 30 years of development, and operating experience with several hundred boilers. With their high availability, they represent the current state of the art. The only criticism in comparison with vertical tubing is the higher cost of manufacturing and installation due to welding of the support straps and the numerous field welds required. Global efforts therefore target replacement of the spiral-wound tube configuration with vertical tubes. The vertically-tubed furnace with Low Mass Flux design was not feasible until Siemens performed further development of rifled tubes to give improved heat transfer. Fig. 7 shows why heat transfer in a rifled tube is so good, especially during evaporation: Centrifugal force transports the water fraction of the wet steam to the tube wall. The resulting wall wetting causes excellent heat transfer from the wall to the fluid. This has the following advantages over smooth tubes: Steam quality Fluid Rifled tube Smooth tube Pressure: 150 bar Mass flux: 500 kg/m 2 s Heat flux: 300 kw/m 2 Rifled tube Smooth tube Inside wall temperature ( C) Figure 7 Wall temperatures and boiling crisis of tubes No deterioration of heat transfer even in the range of high steam quality Very good heat transfer even at low mass flux Only slight increase in wall temperature in case of film boiling near critical pressure (interval from about 200 bar to critical pressure) Potential for increased heat transfer by optimization of rifling geometry. Changes in rifling geometry permit significant improvements in heat transfer to be achieved (Fig. 8). A Siemens high-pressure test rig the largest in the world, with an electrical heater capacity of more than 2,000 kw was used to generate more than 150,000 data points in an investigation of standard commercial rifled tubes and of tubes with modified rifling geometry. For unchanged inner wall temperatures it proved possible to reduce the mass velocity by approximately 25% compared to the use of standard commercial rifled tubes. 7

8 The Low Mass Flux design mentioned in the foregoing not only enables downward extension of the output limits for vertical tubes to 300 or 200 MW and use of large-diameter tubes, but in particular it also changes the flow characteristic of a once-through system: With increased heating of an individual tube, the throughput of that tube increases instead of decreasing. Figure 8 Optimized rifled tubes reduce wall temperatures or allow mass flux reduction This flow behavior well known from drum boilers is called a natural circulation or positive flow characteristic. The standard flow characteristic of once through boilers, where excess heating impedes the flow in individual tubes, is transformed into a natural circulation characteristic when the full-load mass flux is reduced to the value known from drum boilers.for an evaporator tube with 25% higher heat input than an average tube this transition is plotted in Fig. 9 as a function of mass velocity. The example is valid for a supercritical 600 MW BENSON boiler with vertically tubed furnace. A natural circulation characteristic is established below a mass flux of roughly 1,050 kg/m 2 s. Below this value the increase in outlet temperature is more or less compensated by a corresponding increase in mass flux in the individual tube. At values above 1050 kg/m 2 s the throughput decreases with increasing heat input and the outlet temperature increases disproportionately. The theoretical conclusions for this concept regarding flow distribution with non-uniform heating were tested in practice in the supercritical 320-MW FARGE plant. A m high furnace heat exchange surface of Low Mass Flux design was in trouble-free operation at the FARGE plant for more than 10,000 hours. This confirmed the calculation fundamentals and at the end of trial operation the tubes were still practically as good as new, with the rib profile not smoothed by deposits. In addition, the thermohydraulic principles of Low Mass Flux design have already been proven in commercial operation in the horizontal heat recovery steam generator at the COTTAM GUD combined-cycle power plant. The parallel tubes of the BENSON evaporator for the HP and IP stages arranged sequentially in the exhaust flow path are characterized by extremely different heat uptakes to which the mass flow automatically adjust (Fig. 10). 8

9 Figure 9 Mass flow characteristic of vertically tubed BENSON boilers The characteristics of the vertically tubed furnace can be summarized as follows: Mass flux reduction from 2000 to 1000 kg/m 2 s flow characteristic as in drum boilers: increased heat input to an individual tube increases throughput in that tube Cost-effective fabrication and assembly Minimum BENSON output: 20% Simple startup system for 20% evaporator throughput Reduced slagging on furnace walls Figure 10 Heat and mass flow distribution in the HP evaporator of a once-through HRSG 9

10 Horizontal Furnace The scientifically founded design basis, the results of the practically oriented large-scale test and the operating experience with the BENSON heat recovery steam generator at the Cottam CCPP have enabled the establishment of a solid knowledge base for the next step of innovation, the horizontal furnace boiler (HF-boiler). In the HF-boiler, the convection section with the horizontal and vertical passes is located in the gas path downstream of the horizontal furnace, and is largely identical with proven two-pass boilers (Fig. 11). The vortex burners are shown here in a front configuration. HF-boiler evaporator tubes are vertical. This concept is characterized by a very low building height. The potential reduction in structural steelwork, connecting lines to the turbine as well as installation costs and time in comparison with traditional single and two-pass designs is clearly evident. Figure 11 Steam generator with horizontal furnace for 350 MW Studies for the New Concept Concentrate on Four Aspects: Firing: NOX formation and unburned carbon must be evaluated. In addition, the horizontal flame orientation may effect heat flux distribution and thus have an impact on the flow design of the furnace walls. Furnace walls: The evaporator tubes can only be vertical. The influence of the large heat input differences of 3:1 and more over the length of furnace on the outlet temperature of the evaporator tubes connected in parallel must still be investigated. Transition from furnace to horizontal section: Large temperature differences occur here between adjacent tubes and tubes which are welded together. Reinforcement of surrounding walls needs attention because of relatively large spans. The problem of the water/steam side design of the furnace walls is characterized by the fact that an evaporator tube in the main combustion zone has a heat input roughly 3 times as great as at the furnace outlet. Despite this unfavorable situation, the outlet temperatures from the individual parallel tubes must under no circumstances exceed either allowable maximum values 10

11 or set tolerance limits from other tubes. Once again, as already seen in case of HRSG Cottam the Low Mass Flux design provides the solution to this problem (Fig. 12). The natural circulation characteristic described earlier results in a higher mass flux and thus greater throughput in a tube with higher heat input than in a tube with lower heat input. This behavior is further supported by different dimensioning of the tubes and fins. Fig. 12 also illustrates the effect of this design on the tube outlet temperature: The temperature difference between any two tubes is less than 50 K, with that between adjacent tubes even less, below 30 K. Tube throughput also automatically adjusts to new heating conditions, such as a change in heat input due to fouling or soot blowing. Figure 12 Mass flow distribution adjusts automatically to heat flux distribution A boiler with a horizontal furnace is only about 30 m high, which allows simple and fast installation. The heights of comparable conventional steam generators are between 60 m and 90 m (Fig. 13). The advantages of this horizontal, low-profile design are obvious: Reduced cost of structural steelwork Straightforward installation Short installation time due to parallel installation of the furnace, lateral pass and vertical pass Shorter steam lines between boiler and turbine. The modular design of the boiler with its furnace, lateral pass and vertical pass also makes adaptation to other power ratings and other fuels a relatively simple matter. A 700-MW plant with dual furnace, for example, has only twice the width but otherwise the same dimensions as a 350-MW boiler. 11

12 Summary The requirements for modern power plants with regard to efficiency and operating behavior necessarily lead to the implementation of once-through boilers. On the one hand, this boiler type enables supercritical steam conditions, while on the other it is especially suitable for variable-pressure operation. The further development by Siemens of the BENSON once-through boiler with vertical-tube furnace walls constitutes a step in the direction of simpler and thus more cost-effective design with improved operating behavior. The development is based on extensive fundamental research in heat transfer and pressure drop in rifled tubes. Optimization of the rifled tubes enables a significant reduction in mass flux and hence a furnace wall flow design yielding increased throughput in tubes with higher heat input. This behavior is known from natural circulation evaporators. In addition to the cost reduction, the reduction of the minimum BENSON output to 20% is especially attractive, as this makes night and weekend shutdowns unnecessary in many cases. Tower Two Pass Horizontal Figure 13 Size comparison of coal-fired steam generators with horizontal furnace for 550 MW output The development of the HF boiler focused on reducing investment costs by incorporating the operational advantages of vertical tubes. Investigations of possible problem areas were initiated and solutions found. The HF boiler will constitute a milestone in boiler construction for manufacturers and operators alike. The cost advantages continue to increase with increasing steam conditions. 12

13 This paper is based on a lecture held at Power Gen 2000 Published by and copyright 2000 Siemens Power Generation Freyeslebenstraße Erlangen, Germany Phone: contact@erl11.siemens.de Siemens Westinghouse Power Corporation The Quadrangle 4400 Alafaya Trail Orlando, FL (USA) Phone: Two names one global company s Subject to change without prior notice Printed on paper treated with chlorine-free bleach Siemens Aktiengesellschaft Order No. A96001-S90-A703-X-7600 Printed in Germany M VU SEK 22209